1. Field of the Invention
The present invention relates to methods and apparatus for circulating and dispensing fluids; more particularly, to methods and apparatus for circulating and dispensing fluids having magnetorheological properties; and most particularly, to methods and apparatus for managing and metering magnetorheological fluids being used in a magnetorheological finishing apparatus.
2. Discussion of the Related Art
It is well known in the art of finishing and polishing surfaces to use, as a finishing agent, particulate fluid suspensions having magnetorheological properties. Such fluids, known as magnetorheological fluids (MR fluids), comprise magnetically soft particles which can become oriented and magnetically linked into fibrils in the presence of a superimposed magnetic field, thereby increasing the apparent viscosity of the fluid by many orders of magnitude. Such increase is known as magnetic “stiffening” of the MR fluid. It is further known to incorporate finely-divided abrasives into MR fluids used in finishing and polishing to increase the rate of removal of material. Non-stiffened, or magnetically relaxed, MR fluid can be stored and pumped as a low-viscosity fluid, having a viscosity typically of about 50 cp or less, then stiffened to a semi-rigid paste of 105 cp or more in a magnetic work zone for finishing or polishing, then relaxed again outside the work zone for collection, reconditioning, and reuse. Apparatus and methods for magnetorheological finishing and for delivery of MR fluids are disclosed in, for example, U.S. Pat. No. 5,951,369 issued Sep. 14, 1999 and U.S. Pat. No. 5,971,835 issued Oct. 26, 1999, both to Kordonski et al., the relevant disclosures of which are herein incorporated by reference.
MR fluid finishing apparatus typically includes a fluid delivery system (FDS) for dispensing MR fluid onto a rotating carrier surface, whereon the fluid is carried into and out of the work zone. MR fluid is a relatively unstable suspension because the magnetic particles tend readily to agglomerate and to settle out of suspension and thereby stagnate. Thus, a primary concern in configuring an FDS for MR fluid is keeping the fluid relatively homogeneous in the system, and very highly homogeneous at the point of dispensing into the work zone. An FDS must receive spent fluid from the work zone, recondition the fluid for reuse as by adjusting the temperature and viscosity, homogenize the adjusted fluid, and redispense the fluid into the work zone at a controlled flow rate. A suitable prior art FDS is disclosed in U.S. Pat. No. 5,951,369 incorporated above.
Because of these various requirements, the prior art FDS is relatively complex and includes a first peristaltic pump for removing spent fluid from a scraper at the work zone and returning the fluid to a reservoir; a mixer in the reservoir for rehomogenizing the fluid; a tempering subsystem at the reservoir for cooling the fluid, which tends to become heated in the work zone; a second peristaltic pump and cylindrical nozzle having a fixed restriction for redispensing the fluid; a pulse-dampener for removing pulses generated by the pumps; and a viscosity measuring and correcting subsystem. Flow may be controlled by manually setting the speed of the second pump, and preferably is monitored via a magnetic induction flowmeter.
Several problems are presented by the prior art FDS.
First, the system is cumbersome, as it is essentially an assemblage of discrete components, each intended to perform a single task. Thus, the system is wasteful of space.
Second, the flow control system requires a positive-displacement (PD) pump. Some known PD pumps such as gear pumps are unsuited to the task of pumping MR fluids. A peristaltic pump can meet the positive-displacement need over a short period of time; however, the pulsating output mandates the pulse-dampening apparatus already noted, and the delivery lines within the pump are subject to fatigue and must be replaced frequently.
Third, correct composition of the MR fluid being redispensed is inferred from an inline viscometer which incorporates a cylindrical nozzle that, for flow reasons, must be relatively long and thus is cumbersome. In the flow and composition control strategy employed, a constant input pressure at the entrance to the nozzle and a constant flowrate at the flowmeter indicate a constant viscosity and hence constant composition of the fluid being dispensed.
What is needed is an improved fluid delivery system for managing MR fluid in an MR finishing apparatus wherein flow is inherently smooth, pulsations are not generated, and pulsation dampening is unnecessary; wherein the dispensing flow is maintained at a desired flowrate by a closed-loop flow control subsystem; wherein the composition of the MR fluid is automatically corrected to aim during a reconditioning step; wherein the sizes of components such as a dispensing nozzle are minimized; and wherein mixing, tempering, and pressurizing of MR fluid is performed in a single vessel.
It is a primary objective of the invention to provide a simple, compact fluid delivery system for managing and dispensing magnetorheological fluid for use by a magnetorheological finishing apparatus.